Natural Hybridization and Hybrid Zones

New criteria are proposed for describing cases of natural hybridization that overcome the difficulties inherent in schemes based on the history of an interaction (primary or secondary contact) or the taxonomic status of the interacting forms. On the basis of distributional data alone the following types of natural hybrid zones may be recognized: allopatric (corresponding to zones of intergradation), parapatric, peripheral sympatric, localized sympatric, and widespread sympatric. Artificial (extralimital) sympatric hybridization also occurs in nature.